Method of producing biphenyl



Filed NOV. 18. 1950 Feb. 15, 1955 J. H. SAUNDERS ET AL 2,702,307

METHOD OF PRODUCING BIPHENYL -3 Sheets-Sheet l INVENTOR. Jfl/JAS H. sea/wees 05547 J. .szaco/mss E OQ M United States Patent METHOD OF PRODUCING BIPHENYL James H. Saunders, Anniston, Ala., and Robert J. Slocombe, Dayton, Ohio, assignors to Monsanto Chemical Company, St. Louis, Mo., a corporation of Delaware Application November 18, 1950, Serial No. 196,448

17 Claims. (Cl. 260-670) The present invention is directed to a novel method of producing biphenyl by the pyrolysis of benzene.

One object of the invention is to provide an economically and commercially feasible method of producing biphenyl in a continuous manner.

Another object of the invention is to provide a method of making biphenyl which is characterized by great flexibility in that within certain limits any desired conversion to biphenyl and related higher boiling compounds can be obtained.

Another object of the invention is to provide a method of producing biphenyl and related higher boiling hydrocarbons in which localized over-heating which leads to the formation of excessive amounts of carbon is substantially completely eliminated.

Another object of the invention is to provide a method of making biphenyl and related higher boiling hydrocarbons in which the reaction conditions and products are readily reproducible and, therefore, translation from a plant of a given productive capacity to one of a substantially different productive capacity can be easily made without resorting to extensive experimentation.

An additional object of the invention is to provide a method of making biphenyl and related higher boiling hydrocarbons whereby large volume production of the above products is obtainable Without the use of numerous large and expensive pieces of equipment.

A further object is to provide a method of making biphenyl by the pyrolysis of benzene whereby the above product is produced in very high overall yields.

A still further object is to provide a method of making biphenyl whereby efiicient heat conservation is achieved even at low conversion rates and in spite of the fact that the benzene must be recycled a relatively large number of times in order to produce a high overall yield of biphenyl.

Other objec s and advantages will be apparent to those skiltlied in the art as the description of the invention procee s.

In the production of biphenyl from benzene as heretofore practiced, two general methods have been employed, namely, the lead-pot and the tubular methods.

In accordance with the lead-pot method, vaporized benzene is passed through a molten lead bath at a temperature at which substantial quantities of biphenyl are formed whereupon the heated vapors are cooled to condense same and then the resulting condensate is distilled to effect a separation of the biphenyl. This method has enjoyed a large measure of success but has numerous disadvantages.

For example, the productive capacity of the foregoing method is restricted to that of the lead-pots, and, therefore, a great number of large and expensive units are required in order to achieve large volume production.

Moreover, the heat balance in the lead bath method is very poor; in fact, since the maintenance costs of heat exchangers are so excessive on account of the deposit of lead and carbon in the tubes, no attempt is made in actual practice to recover the sensible heat of the vaporous reaction product.

Furthermore, the reaction temperature control in the above method is not entirely satisfactory, and, therefore, the vapors are constantly exposed to overheating which leads to the excessive formation of carbon. Apart from the losses of benzene thereby incurred and the resulting reduction in overall yield of biphenyl, the formation of Patented Feb. 15, 1955 carbon is one of the principal sources of maintenance, necessitating frequent interruptions in operation so that carbon accumulations obstructing the apparatus can be removed.

In the production of biphenyl by the tubular method, benzene vapors are subjected to pyrolysis by passing same through electrically heated carbon or graphite tubes maintained at a temperature between 650 C. and 950 C. The velocity of the gas flow is controlled so that between 5% and 20% of the benzene is pyrolyzed in a single pass. The reaction products are condensed and separated from the residual unreacted benzene by fractional distillation and the latter is then recycled for re use in the method.

In the commercial practice of the tubular method, numerous operating difiiculties are encountered due to inadequate temperature control which results in the gradual accumulation of carbon in the tubular apparatus. This formation of carbon constricts the bore of the tubes, lowers the elficiency of the heat transfer to the benzene vapors, disturbs the balance between the gas flow and the electric current and eventually causes stoppage of the vapor flow if the obstruction is not removed. Consequently, it has been found necessary to periodically shut down the apparatus for the purpose of removing carbon form the tubes. These periodic interruptions in operations add considerably to operating cost and substantially reduce the productive capacity of the plant.

The carbon deposits are formed not only in the electrically heated converter tubes, but also in the metal tubes which convey the benzene vapor feed and carry off the reaction products. In addition to the above carbon deposits, loose carbon is also produced which accompanies the reaction vapors and contaminates the condensate obtained therefrom. Such carbon, irrespective of its method of formation, represents a loss to the method, and, therefore, it is desirable to inhibit its formation as far as possible.

We have developed a method of producing biphenyl by the pyrolysis of benzene, in which the above objectionable features are either completely eliminated or substantially reduced. In accordance with this method, benzene vapor is preheated to a temperature within the range of 500 C. to 800 C. (773 K.-l073 K.) and then subjected to pyrolysis by passing same through an electrically heated tubular reactor maintained at a constant temperature within the above-mentioned limits. The reacted vapors pass in heat exchanging relationship with the liquid benzene feed and are partially cooled, after which they are further cooled to condense the liquefiable constituents such as benzene, biphenyl and related higher boiling hydrocarbons, and the residual gases including hydrogen are vented. The liquid condensate is then fractionally distilled to successively separate unreacted benzene and biphenyl, leaving the higher boiling related hydrocarbons as a residue. The unreacted benzene, after being mixed with fresh benzene, is continuously recycled to form additional quantities of biphenyl, hydrogen and related higher boiling hydrocarbons.

During the pyrolysisreaction, the reaction temperature and the sojourn time of the reaction mixture in the constant temperature reactor are correlated so as to satisfy the following relationship:

wherein it is the reaction velocity constant, t is a sojourn time of the reaction mixture in the constant temperature reactor of from 1 to seconds, a is the initial concentration of the benzene, x is the amount of benzene decomposing in time I, A has values of 4X10 and 1.1 X 10 when E has a value of 45,738 cal/mole K. and 49,006 caL/mole K., respectively, e is the base of the natural logarithm series, R is the gas constant having a value of 1.986 and K is the absolute temperature.

In addition to satisfying the above relationship, it is desirable to correlate the diameter of the reactor tube,

Reynolds number=- where d is the diameter of the reactor, u is the vapor velocity, p is the vapor density and U is the vapor viscosity. In general, Reynolds numbers of about 2,700 and below indicate streamline vapor flow, whereas those above 2,7003,000 indicate turbulent flow.

By correlating the conditions of constant reaction temperature, sojourn time and Reynolds number in the manner described above, the formation of carbon is substantially eliminated. Moreover, the conversion of benzene to biphenyl per pass is thereby accurately controlled so that a product yield of any desired value within the limits of 1% to 30% of theory is obtained and as a corollary thereto, the yield of related higher boiling hydrocarbons is increased or decreased depending upon the variation in percentage conversion per pass.

The relationship between the formation of higher boiling hydrocarbons and biphenyl formation is defined by the following equation:

Sigma ratio= Percent higher boiling hydrocarbons in crude biphenyl Percent conversion to crude biphenyl The above ratio is essentially constant and within the limits of the hereindescribed process has a value equal to 1.12:0.10. It can be seen from this ratio that as the percent conversion of benzene to biphenyl per pass increases within the range of 1% to 30%, the amount of higher boiling hydrocarbons increases and vice versa. Thus, by operating under conditions giving a percentage conversion per pass of and recycling the unreacted benzene, an overall conversion of about 93% of theory is obtained. On the other hand, if the conditions are controlled so as to yield 30% conversion per pass and the unreacted benzene recycled, the overall conversion is about 65%.

For a more complete understanding of the present invention, reference is made to the accompanying drawings and the description thereof, it being understood that modifications and variations in the equipment apparent to those skilled in the art may be made as desired without departing from the scope of the invention.

Figure 1 is a partial vertical sectional view through one form of apparatus used in carrying out the method of the instant invention. This view includes the preheater, the constant temperature reactor and the reactorcondenser system.

Figure 2 is an enlarged sectional view of the preheater taken on line A-A.

Figure 3 is an enlarged sectional view of the reactor taken on line BB.

Figure 4 is an enlarged sectional view taken at the junction of pipe 8 and the preheater tube 1.

Figure 5 is an enlarged sectional view taken at support Figure 6 is an enlarged sectional view taken at supports 37 and 37a.

Figure 7 is a graph illustrating the percentage conversion of benzene to crude biphenyl at temperatures at 848 K., 898 K., 948 K. and 973 K. at various sojourn times up to 80 seconds.

Figure 8 is a graph obtained by plotting log k against UK, which graph is used in determining activation energies or the values of E hereinbefore referred to.

Referring to Figures 1-6, inclusive, reference character 1 represents a No. 310 stainless steel preheater tube which was supported by an Alundum pipe 2. This pipe was surrounded by suitable insulation 3 such as firebrick made of diatomaceous earth and its extremities were closed by plugs 4 of the same or equivalent insulating material. The Alundum pipe was also Wrapped with Nichrome ribbon 5 so that it could be heated at the point indicated to prevent cooling of the preheater exit by radiation and conduction. Th? i l$ulation 3 was enclosed in a welded metal casing 6 of rectangular cross section as illustrated in Figure 2.

The coiled preheater tube 1 was 52 feet in length and had an inside diameter of 0.21 inch and at the points indicated, Chromel-Alumel thermocouples 7 were attached so that the true wall temperature of the tube could be measured. At its entrance end, the preheater tube was welded to a pipe 8 and the latter in turn was connected by c o nventional threaded connectors to the benzene feed line 9. The pipe 8 was provided with a solid metal side arm 10 supporting a copper plate 11 which was connected to an electric power line 12. At the exit end, the tube 1 was welded to the preheater exit pipe 13 and the latter carried a copper plate 14 connecting with an electric power line 15.

The preheater tube 1 was heated by passing a current through the tube itself, the power being supplied by lines 12 and 15.

The preheater exit tube 13 was provided with an iron-constantan thermocouple 16 and another thermocouple 16a for respectively measuring the vapor temperature and also the.wall temperature of tube 13 at the preheater exit. The thermocouple 16 was sealed into the tube 13 by a conventional stufiing box 17 and sup ported in the gas stream by six steel cylinders 18. These cylinders also served to block most of the free space in the zone which was at a temperature of 773 K. or above.

The preheater exit tube 13 was connected to a pressure gauge (not shown) by line 19 and to a condenser system (not shown) by line 20, valve 21 and line 22. The above condenser system served as a means of collecting samples of product so that the degree of conversion of benzene to biphenyl in the preheater could be determined.

The preheater exit tube 13 was connected by pipe 23 to a coiled No. 310 stainless steel constant temperature reactor tube 24, the pipe 23 being provided with a thermocouple 25 so that its wall temperature could be measured.

The reactor tube 24 was surrounded by an Alundum pipe 26, which in turn was insulated by firebricks 27 and the latter was enclosed in a welded metal casing 270. This Alundum pipe was wrapped in Nichrome ribbon 28 which was supplied with electric current from a source (not shown) in such a manner as to maintain the reactor tube at a uniform temperature throughout its length. The temperature of the reactor tube Was measured by thermocouples 29 located at the points indicated in the drawing.

The top end of tube 26 was closed by the bottom of the alundum pipe 2 and the lower end of the first mentioned pipe was provided with an insulating plug of firebrick 30 having an opening 31 for the passage of the thermocouple leads (not shown).

The reactor tube 24 terminated in an elongated section 32 which was connected to the reactor exit pipe 33. This pipe was provided with a thermocouple 34 adjacent to its entrance so that its wall temperature could be measured at this point; in addition, it was also equipped with an iron-constantan thermocouple 35 which was sealed into the pipe by means of a stufling box 36 and supported in the gas stream by any suitable means 37 and 37a. As indicated in the drawings, the thelmocouple 35 was supported by a silica tube 37 and a Pyrex glass tube 37a which held the thermocouple in the center of the vapor stream. The glass tube was provided with perforations 37b which allowed the vapors to flow into pipe 33 and finally into the reactor-condenser system.

In contrast to the method of heating used in connection with the preheater tube, the reactor tube was heated throughout its length by radiation from the inner Wall of Alundum pipe 26 which in turn was heated by contact with the Nichrome ribbon 28.

The reactor exit tube 33 was connected to a pressure gauge (not shown) by line 38 and to a condenser system by way of line 39, valve 40 and line 41. The condenser system included two Allihn condensers 42 and 43, a Friedrichs condenser 44, a receiver 45, a condensate meter arm 46, tubes 47 and 48, stopcocks 49 and 50 and a gas vent line 51. The tube 47 served the function of eliminating gas bubbles which formed when using the meter arm 46 alone and interfered with the measurement of the rate of condensation. The tube 48 serving as an overflow permitted the condensate to pass into the receiver 45 when the stopcock 49 was closed and the meter arm 46 was full of condensate. In this system, the measure of the time required to fill the meter arm 46 when stopcock 49 was closed measured the rate of condensation of the crude product.

A condenser system similar to that described in the preceding paragraph and illustrated in Figure 1 was used in collecting the reaction products from the preheater and measuring their rate of condensation.

The No. 310 stainless steel used for the preheater and reactor tubes had substantially the following composition:

In place of stainless steel, the preheater and reactor tubes may be made of mild steel, iron, copper, coppermanganese alloys and equivalent materials. Moreover, instead of single preheater and reactor tubes, a bank or an assembly thereof may be used if desired.

In practicing the invention in the above described apparatus, the preheater and reactor tubes were brought up to temperature by turning on the power supply. Thereupon, the valves 21 and 40 were set so as to give the proper take-off of reaction products from the preheater and reactor respectively.

Upon placing the apparatus in condition for operation, liquid benzene supplied at a predetermined rate was continuously vaporized and conveyed into the preheater where it was heated to a temperature within the limits of 773 K. and 1073 K. The preheated vapor was then continuously split into two streams, one flowing into the preheater-condenser system where the liquefiable components such as biphenyl, related higher boiling hydrocarbons and unreacted benzene were condensed and separated from the gaseous residue and'the other into the tubular reactor. The benzene vapor which entered the reactor was heated or maintained at a constant temperature within the above limits and the resulting products were then conducted to the reactor-condenser system described above where the liquefiable components, that is, the biphenyl, related higher boiling hydrocarbons and unreacted benzene, were separated from the gaseous residue.

The gaseous residue from the above operation included hydrogen, uncondensed hydrocarbons and nitrogen which was used to feed the benzene vapor. This residue was exhausted into the atmosphere, but in practicing the method commercially the residue would be suitably treated to recover its hydrogen content.

The condensates obtained by the above method were analyzed to determine their contents of crude biphenyl, i. e., biphenyl plus higher boiling related hydrocarbons, the analysis being made in the manner hereinafter described.

The condensate from a given run was weighed and the major portion of the benzene removed by atmospheric distillation, this operation being carried out until the residue contained about 50% biphenyl.

The above residue was poured into an evaporating dish and permitted to evaporate to dryness at room temperature. The resulting solid was ground in a mortar, transferred to a tared paper boat and allowed to evaporate to constant weight. Constant weight was considered to be a loss of no more than 0.1 gram in 34 hours. This product was called crude biphenyl.

The percent conversion to crude biphenyl was then calculated from the following equation:

Weight of crude biphenyl 154 Weight of condensate X Percent conversion 30 grams of crude bipheny was subjected to distillation to remove approximately 1 gram of biphenyl, which step also served to remove any compound boiling below biphenyl. The residue was then poured into a tared beaker and the weight of the material determined.

The crystallization point of the material obtained in the manner described in the preceding paragraph was then determined. After this determination was made, the result was compared with the crystallization points of known mixtures of biphenyl and high boiler. As such mixtures show a depression of crystallization point of about 03 C. per 1% of high boiler and as the crystallization point of biphenyl was known, the percentage by weight of high boiler was readily determined.

The following are illustrative examples of typical modes of carrying out the present invention.

EXAMPLE I Benzene vapor was introduced continuously for 28 minutes into the preheater section of the above described apparatus at a total rate of 5.56 lbs. per hour and while under a gauge pressure of 2.5 lbs./in. In passing through the preheater the benzene vapor was heated up to a maximum of l003 K. and while at that temperature it was continuously divided into two streams. One of these continuously flowed into the preheater-condenser system and the other into the constant temperature reactor at a rate of 3.97 lbs. per hour, the feed rate to the reactor corresponding to a Reynolds number of 4340. The vapor stream entering the reactor continuously passed therethrough in a period of 0.85 second and during this time, it was maintained at a constant temperature of 1003 K.

The hot vapors from the reactor were condensed and collected in the reactor-condenser system in the manner described above.

In the above run, the percentage conversion of benzene to biphenyl per pass in the reactor was 1.57%, the total conversion in the preheater and reactor was 7.05%, and the overall yield of pure biphenyl in the reactor was 92% of theory.

EXAMPLE II The procedure described in Example I was followed employing the following conditions.

Duration of run 26 minutes. Maximum gas temperature in preheater. 700 C. (973 K.).

Constant gas temperature in reactor 700 C. (973 K.).

Feed rate to reactor 4.03 lbs. per hour. Sojourn time in reactor 1.76 seconds. Reynolds number (reactor) 4580.

Pressure in reactor, gauge 21.0 lbs./in.

The percentage conversion of benzene to biphenyl per pass in the reactor was 3.28%, the total conversion in the preheater and reactor was 5.36% and the overall yield of pure biphenyl in the reactor was 94% of theory.

EXAMPLE III Duration of run 15 minutes. Miximum gas temperature in pre- 700 C. (973 K.).

eater.

Constant gas temperature in reactor 700 C. (973 K.). Feed rate to reactor 4.37 lbs/hr. Sojourn time in reactor 34.8 seconds. Reynolds number (reactor) 1020.

Pressure in reactor, gauge 50.6 lbs./in.

In the above operation, there was obtained a conversion per pass of benzene to biphenyl in the reactor which amounted to 23.8%, a total conversion in the preheater and reactor of 28.3% and an overall yield of pure biphenyl in the reactor corresponding to 66.5% of theory.

EXAMPLE IV Benzene vapor was continuously charged for minutes to the apparatus as modified in Example III and preheated to a maximum temperature of 948 K. The preheated vapor was continuously divided into two streams, one going to the preheater-condenser system and the other to the reactor at a flow rate of about 4.35 lbs. per hour and under a gauge pressure of zero lbs./in. The flow rate to the reactor corresponded to a Reynolds number of 1050. The preheated vapor flowing into the reactor was continuously conveyed therethrough in a sojourn time of 8.10 seconds and during this time it was maintained at a constant temperature of 948 K.

In this run, the percentage conversion per pass of benzene to biphenyl in the reactor was 6.6%, the total conversion in the preheater and reactor was 7.05% and the overall yield of pure biphenyl in the reactor was 92% of theory.

EXAMPLE V Duration of run 41 minutes.

Maximum gas temperature in pre- 675 C. (948 K.).

heater.

Constant gas temperature in reactor 675 C. (948 K.).

Feed rate to the reactor 3.88 lbs/hour.

Sojourn time in reactor 0.56 second. Pressure in reactor, gauge 1.8 lbs./in. Reynolds number (reactor) 4480.

The results obtained in the above run are set forth below:

Conversion per pass of benzene to biphenyl in the reactor (percent of theory) Total conversion of benzene to biphenyl 1n the preheater and reactor (percent of theory) 1.28 Overall yield of pure biphenyl in the reactor (percent of theory) 98.5

EXAMPLE VI The apparatus and general procedure in this run were similar to that of Example III, but the following operating conditions were employed:

Duration of run 28 minutes.

Maximum preheater temperature 625 C. (898 K.).

Constant gas temperature in reac- 625 C. (898 K.).

tor.

Feed rate to reactor 2.60 lbs./hour. Sojourn time in reactor 63.5 seconds. Pressure in reactor, gauge 50.7 lbs./in. Reynolds number (reactor) 660.

In the above run, the overall yield of pure biphenyl in the reactor was 90%, the conversion per pass of benzene to biphenyl in the reactor was 7.45% and the total conversion in the preheater and reactor was 8.74% of theory.

EXAMPLE VII In this example, the apparatus and general procedure of Example III were employed, but the operatlng conditions were modified in the manner indicated below:

Duration of run 33 minutes. Maximum preheater temperature 575 C. (848 K.). Constant gas temperature in the 575 C. (848 K).

reactor. Feed rate to the reactor 2.32 lbs/hour. Sojourn time in the reactor 75.9 seconds. Pressure in the reactor, gauge 51.2 lbs./in. Reynolds number (reactor) 630.

The above method of operation resulted in an overall yield of pure biphenyl in the reactor amounting to 98% of the theory, a conversion per pass of benzene to biphenyl in the reactor of 1.73% and a total conversion in the preheater and reactor of 1.85% of theory.

EXAMPLE VIII The apparatus used in this example was similar to that of the preceding examples except that the reactor was a 16.7 ft. No. 310 stainless steel pipe consisting of a series of connected pipe turns having an inside diameter of 1 inch.

Benzene vapor containing 2% acetonewas continuously charged for 15 minutes into the above apparatus and thereby preheated to a temperature of 948 K. The preheated vapor was continuously split into two streams, one of which entered the preheater-condenser system and the other into the reactor. The vapor stream which passed into the reactor had a flow rate of 6.67 lbs./hour and was under a gauge pressure of 10 lbs./in. the flow rate corresponding to a Reynolds number of 1610. This vapor stream was continuously passed through the reactor in a sojourn time of 5.93 seconds, during which time it was maintained at a constant temperature of 948 K.

In the above example, the percentage conversion per pass of benzene to biphenyl in the reactor was 13.1%, the overall yield of pure biphenyl therein was 83% and the total conversion in the preheater and reactor was 14.9% of theory.

EXAMPLE IX The procedure of the preceding example was followed In this run, the conversion per pass of benzene to biphenyl in the reactor was 6.42%, the overall yield of pure biphenyl therein was 92% and the total conversion in the preheater and reactor was 7.15% of theory.

The overall'yield of pure biphenyl in the above examples varied from 65.5% to 98.5% of theory, but in terms of" crude biphenyl the yield was essentially 100% of theory in each case.

The foregoing description has been directed to a continuous method of producing biphenyl by the pyrolysis of benzene in which the unreacted benzene was not recovered from the condensate and recycled to the reactor. However, Example X clearly demonstates the feasibility of this method of operation since the unreacted benzene from a similar condensate was converted to biphenyl as readily as fresh benzene. Therefore, it is within the scope of the instant invention to continuously collect and fractionally distill the condensate into fractions containing unreacted benzene, biphenyl and related higher boiling hydrocarbons and then continuously recycle the first mentioned fraction with fresh benzene to the reactor to form additional amounts of biphenyl and related higher boiling hydrocarbons. More particularly, this would. involve a two stage fractional distillation in which unrecated benzene would be continuously separated from crude biphenyl and the latter would be continuously fractionally distilled to separate biphenyl as an overhead fraction and related higher boiling hydrocarbons as a residue. The unreacted benzene would then be continuously mixed with fresh benzene and continuously recycled to the preheater and reactor.

By controlling the reaction conditions so as to convert from 1% to 30% of the benzene to biphenyl per pass and recycling the unreacted benzene in the above manner, an overall yield of biphenyl in the range of about 66% to about of theory will be producedand simultaneously therewith related higher boiling hydrocarbons in the range of about 44% to about 1% of theory will be formed.

In the development of the method of producing biphenyl in accordance with the present invention, the effects of temperature, sojourn time, pressure, Reynolds number and surfacewolume ratio were studied and of these variables, it was found that only the first two have an appreciable influence on the rates of reaction taking place. The influence of these two variables is clearly illustrated in the following tables:

Table I THE INFLUENCE OF SOIOURN TIME, 973 K.

As has been hereinbefore indicated, in order to obtain the desired predetermined conversion of benzene to bi- Reactor Size S i joum G R to Convey auge sec 1' Run No. sec. Re- $5 Press, Feed, 232, a?

Diam., Length, actor p. s. i. g. p. p. h. actor in. ft.

Table II THE INFLUENCE OF TEMPERATURE Reactor Size 1011111 Conver- Run Temp., Time, a gauge i sion, per- N 0. K. see. Reno cent Re- Drialm Lelfltgith, actm. Number p. s. 1. g. p. p. h. actor With respect to Table I, it can be seen that at a constant temperature of 973 K., the percentage conversion of benzene to biphenyl increased from 6.38% to 22.1% as the sojourn time was increased from 3.59 to 27.4 seconds.

Referring to Table II, it will be noted that as the temperature was increased from 848 K. to 973 K., the percentage conversion of benzene to biphenyl was increased from 1.73% to 23.8%. In addition, it will be evident from the foregoing table that as the temperature was increased, the sojourn time required to achieve a given conversion decreased.

The relationship of temperature and sojourn time to the conversion of benzene to biphenyl is graphically illustrated in Figure 7 where percentage conversion is plotted against sojourn times up to 80 seconds for temperatures of 848 K., 898 K., 948 K., and 973 K.

Thus, the above experimental data and the accompanying graph demonstrate in a striking manner that the conversion to biphenyl is directly afiected by an increase in either gas temperature or sojourn time.

Variations in Reynolds number apparently have no direct effect on the extent of the conversion of benzene to biphenyl.

An indirect effect of variations in Reynolds number is the influence of changing the vapor velocity. If solid particles, e. g., carbon are suspended in the gas stream, a decrease in velocity increases the opportunity for these particles to be deposited in the reactor or connected pipes.

Another indirect effect of Reynolds number variations is on the sojourn of the molecules close to the reactor wall. At any given vapor velocity there is a so-called stagnant layer of vapor adjacent to the wall. As the average velocity falls, this stagnant layer becomes thicker. In this layer the movement of the molecules is largely due to diffusion and at very low average vapor velocities, the actual sojourn time of some of the molecules may be significantly longer than the average, leading to high corilversion and carbon formation close to the reactor Wa Furthermore, the heat transfer in a slow moving vapor is not as efiicient as a rapidly flowing one and consequently there is a tendency for local overheating and localized high conversions to occur at low vapor velocities.

Pressure apparently has no effect on the percentage conversion of benzene to biphenyl when the temperature and sojourn time are held constant.

Moreover, variations in surface to volume ratio of the reactor have no significant effect on the rates of reaction taking place.

35 phenyl, it is necessary to correlate the conditions of constant temperature and sojourn time so as to satisfy the following relationship:

wherein k, t, x, a, A, e, E, R and K have the values given earlier in this application. It will now be shown how the values k, A and E are obtained so that the above relationship may be practically applied.

It is apparent from an inspection of the above equation that as soon as the sojourn time and temperature have been selected, it is a simple matter to determine the value of k from the following relationship:

For example, assume that a temperature of 973 K.

and a sojourn time of 20 seconds have been selected, then it will be observed by referring to the graph of Figure 7 that the corresponding percentage conversion or value of x is 19.6%. By substituting the above values of t and x in the above equation, the value of k is found to be 0.0122. In a similar manner, the following values of k corresponding to 973 K. and sojourn times of 2, 4, 8, 12, 20, and 35 seconds respectively were calculated:

The rate constants or values of k may be similarly calculated for any temperature within the range of 773 K. and 1073 K.

Having obtained the rate constants, the corresponding values of E (the activation energies) may be determined by employing the following relationship:

log I0=1og A- The values of log k are plotted against the values of UK as illustrated in Figure 8 and from the resulting graph, the slope of the'line or ratio of is determined directly. Then, by employing the numerical values of log k and UK which define the numerical value of E may be calculated as follows:

CALCULATION OF LOW VALUE OF E Slope= E lb-(*1) 3 E.=-10,000X1.986X2.303 E=45,738

CALCULATION on HIGH VALUE 015 n -E= 10,714X 1.986X2.303 E=49,006

The values of E having been obtained, the corresponding values of A may be derived in the following manner:

E 2.303 log k-2.303 log A- E "2.303RK E log A-log k+-- The following are sample calculations using a temperature of 973 K. and values of k corresponding to 0.020 and 0.011 respectively.

log A= -1.69897+ 4 log A: 1.69897+ 10.3=8.6

log A= 1.95861+l1.01=9.05

in obtaining a predetermined conversion of benzene to biphenyl, the following experimental data are submitted:

COMPARISON OF CALCULATED CONVERSION WITH OBSERVED CONVERSION AT 948 K.

Conversion, Percent Time, See.

Calculated Observed COMPARISON OF CALCULATED CONVERSION WITH OBSERVED CONVERSION AT 973 K.

Conversion, Percent Time, Sec.

Calculated Observed COMPARISON OF CALCULATED CONVERSION WITH OBSERVED CONVERSION AT 898 K.

Conversion, Percent Time, See.

Calculated Observed COMPARISON OF CALCULATED CONVERSION WITH OBSERVED CONVERSION AT 848 K.

Conversion, Percent Time, Sec.

Calculated Observed The above experimental results demonstrate that once the sojourn time and temperature have been selected, the percentage conversion of benzene to biphenyl may be calculated with surprising accuracy. As a corollary thereto, if the temperature and conversion are selected, the required sojourn time may easily be calculated. Finally, if the percentage conversion and sojourn time have been decided upon, the temperature required to give the selected conversion may be determined.

The various conditions of operation which should be employed in the practice of the method of the instant invention will now-be described in detail.

In the production of biphenyl in accordance with the instant invention, the pyrolysis reaction is executed at a temperature within the range of 773 K. and 1073 K. However, it is desirable in commercial practice to carry out this operation at a temperature within the range of from 873 K. to l023 K. and preferably within the range of 948 K. to 973 K.

The Reynolds numbers and values of k, t and x for the above temperature ranges are as follows:

Reynolds number=600100,000

t=1-100 seconds x: l 30% Reynolds number=600100,000

t=1-75 seconds x=1 %30 Reynolds number==600-100,000

t=1-40 seconds at: 1 %28 Although Reynolds numbers as low as 600 have been used with no apparent increase in carbon formation, it is desirable to use Reynolds numbers substantially in excess of 2700. More specifically, While the present invention in its broadest scope contemplates Reynolds num ber Within the range of 600 to 100,000, it is the desiratum to maintain same within the limits of 3000 and 70,000, and preferably within the limits of 20,000 and 50,000.

The pyrolysis reaction is executed under a gauge pressure which may fluctuate within wide limits, i. e., within 13 the limits of about zero to about 100 lbs./in. In general, however, a gauge pressure of from about zero to about 40 lbs./in. is preferred.

The temperature and sojourn time may be regulated as hereinbefore described to yield any desired conversion per pass Within the limits of 1% and 30%. With a relatively low conversion of benzene to biphenyl per pass, a high overall yield of biphenyl and a small amount of high boiler are obtained. On the other hand, with a relatively high conversion of benzene to biphenyl per pass, a relatively low overall yield of biphenyl and a large amount of high boiler are produced. In view of the foregoing, it is evident that the manner of correlating temperature with sojourn time depends upon the nature of the product desired. However, so far as the production of biphenyl is concerned, it is desirable to use a temperature within the limits of 873 K. and 1023" K., and a sojourn time of 1 second to 75 seconds so as to provide a conversion of benzene to biphenyl per pass of from 4% to 6% and an overall conversion of from 92%-93%. Within the above limits, a temperature of 948 K., a sojourn time of 5 seconds, and a conversion of 5% per pass are preferred.

The pyrolysis of benzene to biphenyl may be carried out, as illustrated in Example X, in the presence of promoters. Other promoters within the scope of the 1nstant invention are found in Patent 2,143,509, which is incorporated by reference in the present application.

The present invention in its broadest aspect provides a method characterized by great flexibility in that any desired conversion of benzene to biphenyl within the range of 1% to 30% may be obtained. However, the invention in its more limited and preferred aspect is concerned with a method utilizing operating conditions which effect about conversion of benzene to biphenyl per pass and an overall yield of about 93% of theory. Thus, by operating in this manner, a very substantial improvement in benzene practice is achieved over that obtainable by the lead pot method wherein the overall yield of biphenyl is about 77% of theory.

The operation of the tubular method at about 5% conversion requires that the unreacted benzene be recycled a large number of times to produce a high overall yield, but this can be done economically due to the decreased heat requirements. For example, the production of hiphenyl at 948 K. to 973 K. is readily accomplished in the tubular unit whereas in the lead pot method a temperature of 1073 K. to 1113" K. is required. Moreover, because of the absence of lead and the very low carbon formation, it is practical to use heat exchangers for facilitating the preheating of the benzene feed by the sensible heat of the vapors leaving the reactor. In addition, because of the lower operating temperature, absence of lead and decrease in carbon formation, the maintenance costs of the heat exchangers are substantially decreased.

In view of the above considerations, it is evident that the tubular method of the instant invention is not only much more attractive than the lead pot method from the standpoint of operation but also from the standpoint of lower production costs.

Although the foregoing description has been restricted to the production of biphenyl and the accompanying higher boiling hydrocarbons, it is also Within the scope of the invention to pyrolyze homologues of benzene such as toluene, xylene, ethylbenzene, propylbenzene and mixtures of two or more of these materials to yield homologues of biphenyl and related higher boiling hydrocarbons.

Although the examples described were those in which the gases passing through the reactor were maintained at a constant temperature by the application of heat from the reactor wall to the gas, it is also within the scope of this invention to convert mononuclear aromatic hydrocarbons to biphenyl and related polynuclear hydrocarbons by rapidly preheating same to a predetermined temperature within the range of 773 K. to 1073 K, then passing the reacting gases through an adiabatic reactor, preferably a tubular adiabatic reactor, under controlled conditions of sojourn time and a substantially constant temperature corresponding to the above predetermined preheating temperature, whereby from 1% to 30% conversion of the aromatic hydrocarbons to biphenyl and its homologues is obtained. The method of separating the high boiler and unreacted aromatic hydrocarbons from biphenyl and its homologues is the same as that described in connection with constant temperature reactor.

Since the heat absorbed in the condensation reactions of the kind described herein is very small, the preheated gases Will undergo very little change in temperature on passage through the adiabatic reactor. For example, it has been calculated that when benzene is preheated to 1000 K. and passed through an adiabatic reactor so that 16% conversion to biphenyl takes place, the drop in temperature of the reacting gases will be less than 10 K. In view of this relatively insignificant drop in temperature due to the endothermic nature of the reaction, the temperature may be considered substantially constant and hence the rate constants developed above can be applied with practically the same accuracy as obtained in the constant temperature reactor.

The above modification of the instant invention offers a number of desirable features. Since all of the heat is added to the aromatic hydrocarbon in the preheater stage of the adiabatic process, this type of reactor has the advantage of being more economical to construct, operate and maintain. Moreover, the possibility of overheating in the adiabatic reactor is eliminated, thereby alfording easier control and freedom from undesired decomposition.

The present invention may be embodied in other specific forms without departing from the spirit or essential attributes thereof, and it is therefore desired that the present embodiments be considered in all respects as illustrative and not restrictive, reference being had to the appended claims rather than to foregoing description to indicate the scope of the invention.

What we claim is:

1. The method of producing biphenyl and its homologues, which comprises passing an armoatic hydrocarbon containing a single benzene nucleus through a tubular reactor at a flow rate corresponding to a Reynolds number of from 600 to 100,000 and under controlled conditions of sojourn time and a constant temperature in the range of 773 K. and 1073 K. throughout the length of said reactor whereby from about 0.9% to 30% conversion of said aromatic hydrocarbon to biphenyl and its homologues is obtained, said conditions being defined by the followin g equation:

where k is the reaction velocity constant, t is a sojourn time in said reactor within the limits of 0.56 to seconds, a is the initial mole fraction of said aromatic hydrocarbon, x is the mole fraction of said aromatic hydrocarbon decomposing in time t, A has values of 4X10 and 1.1 X 10 when E has a value of 45,738 cal./mole K. and 49,006 cal./mole K respectively, 6 is the base of the natural logarithm series, R in said equation is the gas constant having a value of 1.986 and K is the absolute temperature.

2. The method of producing biphenyl, which comprises passing benzene through a stainless steel tubular reactor at a flow rate corresponding to a Reynolds number of from 3,000 to 70,000 and under controlled conditions of sojourn time and a constant temperature in the range of 773 K. and l073 K. throughout the length of said reactor whereby from 1% to 30% conversion of said benzene to biphenyl is obtained, said conditions being defined by the following equation:

wherein k is the reaction velocity constant, t is a sojourn time in said reactor within the limits of 1 to 100 seconds, a is the initial mole fraction of said benzene, x is the mole fraction of benzene decomposing in time t, A has values of 4X10 and l.l l0 when E has a value of 45,738 cal/mole K. and 49,006 cal/mole K. respectively, e is the base of the natural logarithm series, R in said equation is the gas constant having a value of 1.986 and K is the absolute temperature.

3. The method of producing biphenyl which comprises passing benzene through a stainless steel tubular reactor at a flow rate corresponding to a Reynolds number of from 20,000 to 50,000 and under controlled conditions of sojourn time and a constant temperature in the range of 773 K. and 1073 K. throughout the length of said reactor whereby from 1% to 30% conversion of said benzene to biphenyl is obtained, said conditions of sojourn time and temperature being defined by the following equation:

wherein k is the reaction velocity constant, 2 is a sojourn time in said reactor within the limits of 1 to 100 seconds, a is the initial mole fraction of said benzene, x is the mole fraction of benzene decomposing in time t, A has values of 4X10 and 1.1)(10 when E has a value of 45,738 cal./mole K. and 49,006 cal./mole K. respectively, e is the base of the natural logarithm series, R in said equation is the gas constant having a value of 1.986 and K is the absolute temperature.

4. The method of producing biphenyl which comprises passing benzene through a stainless steel tubular reactor at a flow rate corresponding to a Reynolds number of from 3,000 to 70,000 and under controlled conditions of sojourn time and a constant temperature in the range of 873 K. to 1023 K. throughout the length of said reactor whereby from 1% to 30% conversion of said benzene to biphenyl is obtained, said conditions of sojourn time and temperature being defined by the following equation:

wherein k has a value of 0.0005 0.1, x is the mole fraction decomposing in time t and has a value within the range of from 0.01 to 0.30 and t is a value varying from 1 to 75 seconds.

5. The method of producing biphenyl, which comprises passing benzene through a stainless steel tubular reactor at a flow rate corresponding to a Reynolds number of from 3,000 to 70,000 and under controlled conditions of sojourn time and a constant temperature in the range of 948 K. to 973 K. throughout the length of said reactor, whereby from 1% to 28% conversion of said benzene to biphenyl is obtained, said conditions of sojourn time and temperature being defined by the following equation:

wherein k has a value of 0.006 to 0.02, x is the mole fraction of benzene decomposing in time t and has a value within the range of from 0.01 to 0.28 and t is a value a varying from 1 to 40 seconds.

6. The method of producing biphenyl, which comprises passing benzene through a stainless steel tubular reactor at a flow rate corresponding to a Reynolds number of 600 to 1000 and under conditions providing a constant temperature of 848 K. throughout the length of said reactor and a sojourn time of from 50 to 80 seconds and thereby converting from 1.3% to 1.9% by weight of said benzene to biphenyl.

7. The method of producing biphenyl, which comprises passing benzene through a stainless steel tubular reactor at a flow rate corresponding to a Reynolds number of 700 to 4700 and under conditions providing a constant temperature of 898 K. throughout the length of said reactor and a sojourn time of 4 to 68 seconds and thereby converting from 1% to 9% by weight of said benzene to biphenyl.

8. The method of producing biphenyl, which comprises passing benzene through a stainless steel tubular reactor at a flow rate corresponding to a Reynolds number of 800 and 4900 and under conditions providing a constant temperature of 948 K. throughout the length of said reactor and a sojourn time of from 1-39 seconds and thereby converting from 1% to 20% by weight of said benzene to biphenyl.

9. The method of producing biphenyl which comprises passing benzene through a stainless steel tubular reactor at a flow rate corresponding to a Reynolds number of from 1000 to 4600 and under conditions providing a constant temperature of 973 K. throughout the length of said reactor and a sojourn time of from 1-37 seconds and thereby converting from 3% to 28% by weight of benzene to biphenyl.

10. The method of producing biphenyl, which comprises passing benzene through a stainless steel tubular reactor at a flow rate corresponding to a Reynolds number of 20,000 to 50,000 and under conditions providing a constant temperature of 948 K. throughout the length of said reactor and a sojourn time of 5 seconds and thereby converting about 5% by weight of said benzene to biphenyl per pass.

11. The method of continuously producing biphenyl and related higher boiling hydrocarbons, which comprises passing benzene through a stainless steel tube reactor at a flow rate corresponding to a Reynolds number of 20,000 to 50,000 and under conditions providing a constant temperature of 948 K. throughout the length of said reactor and a sojourn time of 5 seconds to continuously produce a mixture of biphenyl, related higher boiling hydrocarbons, and unreacted benzene, continuously separating said unreacted benzene from said mixture and continuously recycling same with fresh benzene to said reactor, and thereby obtaining an overall yield of biphenyl of about 92-93% of theory.

12. The method of continuously producing biphenyl and its homologues, which comprises continuously passing an aromatic hydrocarbon containing a single benzene nucleus, which has been preheated to a predetermined temperature in the range of 773 K. to 1073 K., through an isothermal reactor at a flow rate corresponding to a Reynolds number of from 600 to 100,000 and under controlled conditions of sojourn time and a substantially constant temperature throughout the length of said reactor, which is substantially identical, to said predetermined preheating temperature, whereby from 1% to 30% conversion of said aromatic hydrocarbon to biphenyl and its homologues is obtained, said conditions being defined by the following equation:

where k is the reaction velocity constant, t is a sojourn time in said reactor within the limits of 1 to seconds, a is the initial mole fraction of said aromatic hydrocarbon, x is the mole fraction of said aromatic hydrocarbon decomposing in time t. A has values of 4 10 and 1.1 10 when E has a value of 45,738 cal./mole K. and 49,006 cal./mole K. respectively, e is the base of the natural logarithm series, R in said equation is the gas constant having a value of 1.986 and K is the absolute temperature.

13. The method of continuously producing biphenyl, which comprises continuously passing benzene preheated to a predetermined temperature within the range of 773 K. and 1073 K. through a tubular isothermal reactor at a flow rate corresponding to a Reynolds number of from 3,000 to 70,000 and under controlled conditions of so journ time and a substantially constant temperature throughout the length of said reactor, which is substantially identical to said predetermined preheat ng temperature, whereby from 1% to 30% conversion of said benzene to biphenyl is obtained, said conditions being defined by the following equation:

where k is the reaction velocity constant, t is a sojourn time in said reactor within the limits of 1 to 100 seconds, a is the initial mole fraction of said aromatic hydrocarbon, x is the mole fraction of said aromatic hydrocarbon decomposing in time t, A has values of 4X 10 and 1.1)(10 when E has a value of 45,738 cal/mole K. and 49,006 cal./mo1e K. respectively, e is the base of the natural logarithm series, R in said equation is the gas constant having a value of 1.986 and K is the absolute temperature.

14. The method of continuously producing biphenyl, which comprises continuously preheating benzene to a predetermined temperature within the range of 773 K. and 1073 K., continuously passing said preheated benzene vapor through a tubular isothermal reactor at a flow rate corresponding to a Reynolds number of from 20,000 to 50,000 and under controlled conditions of sojourn time and a substantially constant temperature throughout the length of said reactor, which is substantially identical to said predetermined preheating temperature, whereby from 1% to 30% conversion of said benzene to biphenyl is obtained, continuously separating the unreacted benzene from the resulting conversion product, continuously mixing said separated benzene with fresh benzene, continuously preheating the resulting mixture to said predetermined temperature, and continuously recycling said preheated mixture to said reactor, and thereby obtaining an overall where k is the reaction velocity constant, I is a sojourn time in said reactor Within the limits of 1 to 100 seconds, a is the initial mole fraction of said aromatic hydrocarbon, x is the mole fraction of said benzene decomposing in time t, A has values of 4 10 and 1.1 when E has a value of 45,738 cal./mole K. and 49,006 caL/mole K. respectively, e is the base of the natural logarithm series, R in said equation is the gas constant having a value of 1.986 and K is the absolute temperature.

15. The method of producing biphenyl and its homologues, which comprises passing a hydrocarbon containing a single benzene nucleus through a tubular reactor while heating same substantially throughout its entire length at a constant temperature of from 773 K. to 1073 K. and thereby converting from 1% to 30% by weight of said hydrocarbon to biphenyl and its homologues, said convesion taking place in a sojourn time of from 1 to 100 secon s.

16. The method of producing biphenyl which comprises passing benzene through a stainless steel tubular reactor at a flow rate corresponding to a Reynolds number of from 600 to 100,000, While heating same substantially throughout its entire length at a constant temperature of from 873 K. to 1023 K. and thereby converting from 1% to of said benzene to biphenyl, said conversion taking place in a sojourn time of from 1 to 75 seconds.

17. The method of producing biphenyl, which comprises passing benzene through a stainless steel tubular reactor at a flow rate corresponding to a Reynolds number of from 600 to 100,000, while heating same substantially throughout its entire length at a constant temperature of from 948 K. to 973 K. and thereby converting from 1% to 28% by Weight of said benzene to biphenyl, said conversion taking place in a sojourn time of from 1 to seconds.

References Cited in the file of this patent UNITED STATES PATENTS 1,907,498 Carothers May 9, 1933 2,099,350 Stoesser Nov. 16, 1937 

1. THE METHOD OF PRODUCING BIPHENYL AND ITS HOMOLOGUES, WHICH COMPRISES PASSING AN ARMOATIC HYDROCARBON CONTAINING A SINGLE BENZENE NUCLEUS THROUGH A TUBULAR REACTOR AT A FLOW RATE CORRESPONDING TO A REYNOLDS NUMBER OF FROM 600 TO 100,000 AND UNDER CONTROLLED CONDITIONS OF SOJOURN TIME AND A CONSTANT TEMPERATURE IN THE RANGE OF 773* K. AND 1073* K. THROUGHOUT THE LENGTH OF SAID REACTOR WHEREBY FROM ABOUT 0.9% TO 30% CONVERSION OF SAID AROMATIC HYDROCARBON TO BIPHENYL AND IT HOMOLOGUES IS OBTAINED, SAID CONDITIONS BEING DEFINED BY THE FOLLOWING EQUATION: 